My plasma crashed repeatedly while searching for a program from the launcher menu.
Plasma would recover each time with report that Baloo has failed to start.
Restarted twice to see if it solves, no success.
Attempted to reinstall all Baloo related packages, when this "_apt" user issue is coming.

I don't understand what this command should do and it does not work:
# Fixup any mistake in the home directory of the _apt user
if dpkg --compare-versions "$2" lt-nl 1.1~exp10~; then
usermod --home /nonexistent _apt

I also had to delete all files in /var/lib/update-notifier/package-data-downloads/partial/

This bug is obviously NOT a duplicate of a synaptic bug, because it occurs with aptitude, not synaptic. The title is misleading as well, because this bug does NOT concern the file '/root/.synaptic/tmp//tmp_sh', but rather files like '/tmp/aptitude-root.24195:wYCEcM/aptitudeDownload6J+8J:+PsVGmTNm^.^::Lz:%.Hi55VKA' and '/var/lib/update-notifier/package-data-downloads/partial/andale32.exe.'

Please treat this bug as different from the supposed "original". The underlying issue (missing(?) system user _apt and wrong(?) directory owner, group and/or permission) probably is the same, but the directories used are different. Fixing one bug by e.g. correcting one directories permissions will probably NOT fix the supposed "duplicate" bug.

Thanks for this nice workaround, John, but I would strongly suggest not to execute that first line. If you have already, set the permissions back to 755. Instead, the better way to fix this permission problem is the following line:

May I at this point also say that the continued existence of this trivial bug is a disgrace and a reason for me to question if there are structural problems within the core Ubuntu development community that may easily lead to more serious problems.

encountered this today. Resulted in an "information available" window opening that appears forever unsatisfied ("run this action now" button doesn't make it go away, even after applying the fix at bottom)
Information available dialogue is as follows:
Failure to download extra data files
The following packages requested additional data downloads after package installation, but the data could not be downloaded or could not be processed.
ttf-mscorefonts-installer
The download will be attempted again later, or you can try the download again now. Running this command requires an active Internet connection.

The following steps succeeded and installed the troublesome ttf-mscorefonts-installer:

I haven't verified that it creates precisely the user necessary. If I remember correctly, /bin/false means no shell access so make sure the user created does in fact have /bin/false in passwd. Also it has no group (65534). That is it really.

My system already had an _apt user, but did not have proper directory permissions. At least, I have to assume that this is a proper fix. If not, somebody more knowledgeable should be speaking up and sorting this out...

Is there a way to manually installing this package? I get different problems every attempt with the secondary web fetches (XXXXXX32.exe files). Usually it succeeds on 1-4 .exes but it is never able to get them all. The HTTP errors I get are (examples):

Hi, the Debian package works.
Maybe the bug is a wrong folder:
In 3.4 (Ubuntu) I see /usr/lib/msttcorefonts/...
(but it does not exist)
In 3.6 (Debian) I see /var/lib/msttcorefonts/...
(which seems to be correct)

Installing 'apt-transport-https' allows the ttf-mscorefonts-installer to run successfully, but I still get the same "Can't drop privileges for downloading as file . . . couldn't be accessed by user '_apt'" message.